1. Field of the Invention
This invention relates to digital color correction which may be used in color video systems.
2. Description of the Prior Art
In color correction apparatus for adjustment of the primary colors, for example for use in video broadcasting applications, the controls generally include separate adjustments for gain, black level and gamma (described in greater detail below) for each of the three primary colors, namely red (R), green (G) and blue (B). Other controls may also be provided, such as luminance (Y) input gain, chroma input gain and overall output gain. Existing color correction apparatus generally operates in the analog domain. In order to provide a digital implementation of such apparatus, it would be relatively simple to perform gain and black adjustments using digital hardware, since gain adjustment involves multiplication by a factor and black adjustment involves addition of a constant, both of which operations can be readily performed digitally. However, as will now be explained, gamma correction involves a non-linear function and this causes problems for digital implementation.
The characteristic of picture display devices such as cathode ray tubes (CRTs) is non-linear, and it is therefore necessary within a television transmission system to compensate for this non-linearity at the transmitter. A typical input/output relationship for a CRT is shown in FIG. 1 of the accompanying drawings, the characteristic being shown of signal voltage (VS) against light output (LO). The illustrated curve may be described by a power law, as follows: EQU L=K.multidot.E.sup..gamma. ( 1)
where:
L is the light output (LO) of the CRT PA0 K is a constant PA0 E is the signal voltage (VS) PA0 .gamma. (gamma) is a constant.
Hence, as will be seen in FIG. 1 and is apparent from the equation (1), the light output increases more rapidly than the signal voltage.
If the source of the video signals is assumed to be a monochrome camera having a linear input/output characteristic, then the effect of the CRT will be to stretch out changes in luminance in the highlight areas and to compress these changes in the low light areas. Therefore, in order to compensate for these effects, the transmitter must have the inverse law to that of the CRT; the term given to the process that produces this inverse law is gamma correction. An example of the combined effect of a camera and a gamma correction circuit is shown in FIG. 2 of the accompanying drawings, which shows the relationship between the light on the camera tube (LC) and the transmitter output (TO). As will be apparent, the characteristic of FIG. 2 compensates for the characteristic of FIG. 1, and the overall effect is to produce a linear system.
In any particular system, gamma will be a constant, typically between about 2.2 and 2.5. However, as a result of incorrect lighting conditions or the like, it may be necessary to vary the amount of gamma correction to be applied.
In theory, one method of gamma correction would involve hardware implementation of the equation (1). In the analog domain, this could be achieved by using log and anti-log amplifiers with an interposed amplifier whose gain may be varied. Such an arrangement is shown in the block circuit diagram of FIG. 3 of the accompanying drawings. An input signal x is applied to a log converter 10 whose output a is supplied to a gamma adjust amplifier 12. The gamma adjustment is set by the gain of the amplifier 12, and therefore if the gamma adjustment is to be changed, the gain of the amplifier 12 is changed. The output b of the amplifier 12 is fed to an anti-log converter 14 which produces an output signal y.
It will be seen that the following relationships apply. EQU y=e.sup.b =e.sup..gamma..multidot.a =e.sup..gamma..multidot.log x EQU log y=.gamma.log x.multidot.log e EQU log y=log x.sup..gamma. EQU y=x.sup..gamma.
In order to implement the circuit of FIG. 3 in hardware form, it would be necessary to use, for example, arrays of diodes so as to obtain the required log and anti-log characteristics. However, for color correction purposes, it is difficult to match the diode arrays for the three color RGB channels, and also, temperature stability is a problem. In a digital color corrector, the circuit of FIG. 3 could be implemented using programmable read only memories (PROMs) in which the log and anti-log characteristics have been mapped. However, the problem with this method would be truncation of the characteristic in order to enable a sensible word size to be used.